» Magnesium Hydroxide Paste is an aqueous paste of Magnesium Hydroxide. It contains not less than 93.0 percent and not more than 107.0 percent of the labeled amount of magnesium hydroxide [Mg(OH)2], the labeled amount being not less than 28.0 percent and not more than 70.0 percent of magnesium hydroxide.

Packaging and storagePreserve in tight containers.

IdentificationOne g of Paste dissolved in 10 mL of 3 N hydrochloric acid responds to the tests for Magnesium 191.

Soluble alkaliesAccurately weigh a portion of Paste, equivalent to about 7.75 g of magnesium hydroxide, and mix with 75.0 mL of water. Transfer about 25 mL of this diluted Paste to a filter, and reject the first 5 mL of the filtrate. [noteRetain the remaining diluted Paste for the tests for Carbonate and acid-insoluble matter and Heavy metals. ] Dilute 5 mL of the clear filtrate with 40 mL of water. Add 1 drop of methyl red TS, and titrate the solution with 0.10 N sulfuric acid to the production of a persistent pink color: not more than 1.0 mL of the acid is required.

Soluble saltsTo 5.0 mL of the clear filtrate obtained in the test for Soluble alkalies add 3 drops of sulfuric acid, evaporate on a steam bath to dryness, and ignite gently to constant weight: the residue weighs not more than 12 mg.

Carbonate and acid-insoluble matterTo 1 mL of the diluted Paste obtained in the test for Soluble alkalies add 2 mL of 3 N hydrochloric acid: not more than a slight effervescence occurs, and the solution is not more than slightly turbid.

Test preparationTransfer a portion of the Paste, equivalent to 250 mg of Mg(OH)2, to a beaker, add 30 mL of Dilute hydrochloric acid, and stir until dissolved, heating if necessary. Transfer the solution so obtained to a 200-mL volumetric flask containing 4 mL of Lanthanum solution, dilute with water to volume, and mix.

Heavy metals, Method I 231To 4.0 mL of the diluted Paste obtained in the test for Soluble alkalies add 6 mL of 3 N hydrochloric acid, and evaporate the solution on a steam bath to dryness, with frequent stirring. Dissolve the residue in 20 mL of water, and evaporate to dryness in the same manner as before. Redissolve in 20 mL of water, filter if necessary, and dilute with water to 25 mL: the limit is 5 ppm, based on the amount of diluted Paste taken.

Test solutionAccurately weigh an amount of Paste equivalent to 0.25 g of magnesium hydroxide. Cautiously add 3.0 mL of nitric acid, and mix until the sample is dissolved. Accurately transfer this solution to a 50-mL volumetric flask, and dilute with water to volume. [noteThis concentration is recommended if an ICPMS instrument is used. If an ICPAES instrument is used, the concentration of the Test solution may be modified to adapt to the working range of the instrument. ]

ProcedureProceed as directed in the test for Limit of lead under Magnesium Hydroxide. Calculate the content of lead in the portion of Paste taken based on the content of magnesium hydroxide in the Paste, as determined in the Assay: not more than 0.00015% (1.5 ppm) is found.

AssayTransfer an accurately weighed portion of Paste, equivalent to about 250 mg of magnesium hydroxide, to a 100-mL volumetric flask. Dissolve in 10 mL of 3 N hydrochloric acid, dilute with water to volume, and mix. Filter, if necessary, and transfer 25.0 mL of the filtrate to a beaker containing 75 mL of water, and mix. Adjust the reaction of the solution to a pH of 7 (using pH indicator paper; see Indicator and Test Papers under Reagents in the section Reagents, Indicators, and Solutions) with 1 N sodium hydroxide, add 5 mL of ammoniaammonium chloride buffer TS and 0.15 mL of eriochrome black TS, and titrate with 0.05 M edetate disodium VS to a blue endpoint. Each mL of 0.05 M edetate disodium is equivalent to 2.916 mg of magnesium hydroxide [Mg(OH)2].